A power supply controller is conventionally provided, in which a high-power semiconductor switching element such as a power MOSFET is disposed on a power supply line connected between a power source and a load, and which is configured to control the power supply to the load by switching the semiconductor switching element between ON and OFF. Some of such power supply controllers have a self-protective function. The self-protective function turns off the semiconductor switching element in response to an occurrence of overcurrent by controlling the potential of the control terminal of the semiconductor switching element, so as to protect the semiconductor switching element. Specifically, a current detecting resistor is serially connected to a current conducting terminal (e.g., the source or drain in the case of a MOSFET) of the semiconductor switching element, as shown in JP-A-2001-217696, for example. A load current passing through the semiconductor switching element is detected based on the voltage drop on the resistor, and the occurrence of overcurrent is determined if the load current is higher than a predetermined threshold. If the current is shut off due to the above-described self-protective function, the switching element is automatically restored to an ON state when a predetermined time has elapsed since the shutoff. This is because the function is provided to prevent the semiconductor switching element from overheating and, when the abnormal current is shut off, the temperature of the semiconductor switching element should be immediately lowered by a heat radiator conventionally provided therein.
In the case that the load is a motor or a lamp, for example, an inrush current, i.e., a current much higher than the rated current of the load, may pass through the semiconductor switching element for a short time when the power supply controller is powered on. If the above threshold for determining the overcurrent is set to a lower level than the inrush current, for example, the semiconductor switching element will repeat a shutoff/recovery operation, i.e., shutoff due to the inrush current and return to an ON state a predetermined time after that, until the inrush current has died down after the power-on. Then the problem arises that the control is slow to progress to the power supply for the load.
If the threshold is set to a higher level than the inrush current to avoid this problem, another problem arises that an overcurrent can be determined only if higher than the inrush current, when the overcurrent due to short-circuiting in the load, for example, passes through the semiconductor switching element after a steady state is reached.
Thus, there is a need in the art to detect an overcurrent anomaly early while implementing a measure against an inrush current.